po-erase

A paint operation that "erases" the color that is being drawn. More
precisely, each bit in the resulting color value of a pixel on the
destination stream will be set to one if the corresponding bit was
already one on the destination stream but zero in the new content
being drawn. See paint-operation.

For example, if you set the foreground-color of a stream to be red and set its paint-operation to be po-erase, and then you draw a
line on the stream, then the redness will be removed from whatever
colors had appeared on the stream where the line was drawn.

If the stream has a palette,
then the bit-combining is done on palette indices rather than on
true-color values. This tends to result in palette indices of
unrelated colors that are not predictable from the colors being
combined.